Handover is a vital part of a typical cellular communication system. A handover is the process of transferring an ongoing connection of a mobile wireless communication device from one (serving) cell to another (target) cell in order to accomplish transparent service over a geographical area that extends over more than one cell. Ideally, the handover should be performed without any loss of data and with as small communication interrupt as possible. Handover decisions are made by one or more network nodes of the cellular communication system. Handovers for the uplink (UL) and the downlink (DL) may be performed together or separately. To provide an illustrative description, this disclosure will focus on the example of downlink handover.
Enabling a handover typically comprises finding a suitable target cell and ensuring that it is possible to sustain reliable communication with the target cell. Candidates for suitable target cells are usually stored in a so-called neighboring cell list which may, for example, be stored at the network node providing the serving cell. Finding a suitable target cell and ensuring that it is possible to sustain reliable communication with the target cell typically comprises comparing current quality properties (e.g. received signal power/strength or similar) of the cells in the neighboring cell list and selecting the target cell according to some criteria (e.g. highest signal power/strength, possibly with a hysteresis).
The current quality properties of a target cell are typically estimated via measurements associated with the downlink of the wireless communication device that is prospectively being handovered. Using downlink measurements for handover decisions typically require that reference symbols are provided in the downlink, that the wireless communication device performs the measurements, and that measurement reports are transmitted in the uplink. Thus, considerable signaling and processing resources are needed for the handover based on downlink measurements to work.
A conceptually more attractive approach is to base the target cell selection for downlink handover on measurements associated with the uplink of the wireless communication device that is prospectively being handovered. According to such an approach, the node providing the serving cell and one or more nodes providing prospective target cells perform measurements of signals from the wireless communication device (e.g. sounding signals, data reference signals). Results of the measurements are reported to a network node which controls the handover (typically the node providing the serving cell) and handover decisions are based on the measurement results.
Such an approach may be particularly attractive in cellular communication systems employing narrow beamforming implemented with advanced antenna systems, wherein handover between the beams (provided by the same network node or by different network nodes) may be necessary. For example, for handover between beams provided by different network nodes, where a wireless communication device would typically have to perform and report measurements for all (of a very large number of) possible beam configurations if handover decisions are based on downlink measurements, the approach of using uplink measurements for handover decisions seems very attractive.
This approach, however, only works robustly if the quality property relation between the uplinks of the considered cells is the same as (or at least very similar to) the quality property relation between the downlinks of the considered cells, which is not always the case. This is not true, for example, in heterogeneous networks where the downlink transmission power typically differs between different network nodes.
Therefore, there is a need for improved handover control methods and arrangements wherein handover decisions are based on uplink measurements.